Prefoldin 4 (PFDN-4) is a 134 amino acid protein having a helix-loop-helix domain. The protein is a subunit of the heterohexameric chaperone protein prefoldin, a cytoplasmic protein that assists in the correct folding of other proteins, such as actin and tubulin, which are major components of the cellular cytoskeleton. Human PFDN-4 has been localized to chromosome region 20q13. This region is frequently amplified in a variety of human cancers, including breast, ovarian, and cervical cancer. The sequence of the breast cancer amplicon has been analyzed (see, e.g., Collins et al., Genome Res. 11:1034-1042, 2001). Although PFDN-4 was identified as one of the genes present in the amplified region and was one of the genes shown to be overexpressed in cell lines in which the amplicon was present, there was no evidence that PFDN-4 played a role in tumorigenicity.